When grandparents lived long enough to pass on the knowledge they gained from extended lifespans, modern humans made a huge evolutionary leap, according to research conducted by two anthropologists.
Wear patterns on 750 dental remains from four different types of hominids--ranging from three-million-year-old australopithicenes to modern humans around 18,000 years old--showed that the number of people who died when they were "old," defined as 30, or twice the age of reproductive maturation, gradually increased over millions of years. But the ratio of older to younger adults skyrocketed in the Upper Paleolithic period, increasing fivefold around 30,000 B.C. With more people living longer, and in the process producing more children, a population boom followed, as did major changes in human behavior, primarily symbolic thinking.

Researchers Rachel Caspari of the University of Michigan and Sang-Hee Lee of the University of California at Riverside say the results back up the so-called Grandma Hypothesis, which proposes that older women's knowledge and care helped ensure the survival of daughters' offspring. "When you have a lot of people, you have the transmission of information," says Caspari. "There are more ideas to bounce around." Caspari also suggests that menopause--a human characteristic unique among primates--may have developed in this era, with women beyond their childbearing years having the time and the expertise to lend a hand to child-rearing responsibilities in the group as a whole.